The present invention relates to a multi-color printing machine, and more particularly, to an apparatus for correcting a skew of a picture in a multi-color offset printing machine.
Multi-color offset printing presses are generally disclosed in U.S. Pat. No. 3,384,011 and DE-OS 2,237,567.
Conventionally, various attempts have been made to correct or compensate a skew or slant of a picture to be printed, especially in a multi-color printing machine. For example, U.S. Pat. No. 4,362,098 discloses a twisting means for adjusting the relative position between a plate cylinder and an impression cylinder. With the disclosed structure, an image may be distorted and a uniform image development may not be obtainable along both longitudial and lateral directions of the imaging medium.
According to a picture adjusting device in a sheet-fed press disclosed in U.S. Pat. No. 4,777,876, a printer includes two printing sections (one for each color) and a base impression cyinder. Each printing section includes a plate cylinder, and an inked impression cylinder. These cylinders are all mounted parallel to one another. A paper-feed cylinder is provided to transfer paper sheets from a paper feeding section to the base impression cylinder for printing. The paper-feed cylinder is mounted at one end with a bearing held in an adjustable eccentric holder so that the rotation axis of the paper-feed cylinder is adjustable relative to the rotation axis of the impression cylinder. Accordingly, the printing position of the paper can be adjusted relative to both the inked impression cylinders. That is, the picture position can be adjusted merely by adjusting the eccentric holder. As a result, it becomes easy to control the alignment of two-color images printed in succession with two printers.
The picture adjusting device includes a frame, the rotatable paper-feed cylinder and the bearing means. The base impression cylinder and the plate and inked impression cylinders are rotatably supported on the frame. The rotatable paper-feed cylinder is separate from the base impression cylinder and the plate and inked impression cylinders, and is adapted for feeding the paper to the impression cylinder. The bearing means is adapted for supporting the shaft of the paper-feed cylinder at its opposite ends by the frame. At least one of the bearing means includes the eccentric holder movable in a plane substantially perpendicular to the shaft. As a result, the paper-feed cylinder has a rotation axis tiltable relative to a rotation axis of the base impression cylinder to adjust a placement of a sheet of paper fed to the base impression cylinder relative to the plate and the inked impression cylinders.
Here, the eccentric holder is angularly moved or turned by means of an operation knob for twisting the paper-feed cylinder with respect to an impression cylinder. However, according to such structure, delicate and accurate picture image adjustment cannot be achieved, since the amount of twist of the paper-feed cylinder and the amount of change in distance between the axis of the paper-feed cylinder and the axis of the impression cylinder become excessively large due to the rotational manupulation of the opreation knob.
According to a modification, a bearing is slidably received in a groove formed in the frame and can be fastened to the frame by a screw at an adjusted position via a twisting means. However. the latter apparatus also has a problem in that a fine adjustment may not be achievable.